Various gypsum products, including wall panels, ceiling panels and tiles, are commonly used in the construction industry. Many of these gypsum products are made by preparing an aqueous gypsum slurry with calcined gypsum (calcium sulfate alpha hemihydrate, calcium sulfate beta hemihydrate and/or calcium sulfate anhydrate), shaping the slurry and then allowing the slurry to harden as calcined gypsum rehydrates into gypsum (calcium sulfate dihydrate). A gypsum board composite, known as wallboard, can be manufactured by sandwiching a gypsum slurry between two paper cover sheets.
As described in detail in U.S. Pat. No. 4,959,272, the entire disclosure of which is incorporated herein by reference, paper for a wallboard cover sheet is conventionally made by pulping up with water various cellulosic fiber materials which may comprise waste paper including old corrugated paper, kraft cuttings and waste newspaper. This paper-water stock is then formed into paper plies by draining water on several continuously moving wire cylinders. In order to make a multi-ply paper, separate paper plies are superimposed and joined together by a carrying felt. The weak paper web is then dewatered in a press section where water is pressed out of the web. The pressed paper is dried in a multi-cylinder drying section with steam added to each cylinder. The dried paper is subjected to a squeezing or calendaring operation for uniformity in thickness and is then finally rolled into rolls. This paper is subsequently utilized as paper cover sheets to form gypsum wallboard by depositing a calcined gypsum slurry between two sheets, and permitting the gypsum to set and dry.
Typically, two different paper cover sheets are used. One paper cover sheet is used for the back side of the wallboard. This paper cover sheet is referred to as the backer paper sheet. Another paper cover sheet is used for the face side of the wallboard which will be visible and decorated after the wallboard installation. This paper cover sheet is referred to as the facer paper sheet.
A multi-ply paper cover sheet may comprise different plies. Several filler plies, typically from 2 to 5, are prepared of a hard stock nature such as for example, kraft and boxboard cuttings. The set of filler plies may then be overlaid with 1 to 2 top liner plies, also referred to as liner or outer plies. Typically, a liner ply consists mainly of flyleaf shavings and newspaper of a soft ground wood quality with shorter fibers. A wallboard is covered with a laminated multi-ply paper cover sheets. Typically, paper plies are adhered to each other by being superimposed over each other right after the ply formation and subsequent pressing.
In wallboard, paper cover sheets are contributing to the nail pull strength, flexural strength, and water and mold resistance. Studies have also revealed that the tensile strength of a paper cover sheet can influence the fire performance of gypsum fire-rated products.
Efforts have been made to prevent separation of paper plies from each other in a laminated multi-ply paper cover sheet as well as from splitting a paper ply directly adjacent to a gypsum core. As provided in U.S. Pat. No. 2,806,811, a resinous additive can be added to a paper pulp during making a paper ply as furnish. This method is also referred to as internal addition because the additive with very low usage is mixed directly with pulp during formation of a paper ply so as to minimize the white water chemistry change. However, improving the paper tensile strength continues to be a subject of ongoing research in the paper industry.